(1) Field of the Invention
The present invention relates to a semiconductor device having, on a semiconductor substrate, a memory section having an ONO (an upper silicon oxide layer/a silicon nitride layer/a lower silicon oxide layer) film as a gate dielectric and a logic (CMOS) section and a method for fabricating the same.
(2) Description of Related Art
With advances in miniaturization of elements, increasing significance has been attached to semiconductor devices in which a memory section and a CMOS section are formed on the same semiconductor substrate and which use an ONO film as a gate dielectric of a memory transistor constituting a part of the memory section.
A known semiconductor device in which a memory section and a CMOS section are combined and a method for fabricating the same will be described hereinafter with reference to FIGS. 7A through 8C.
First, as shown in FIG. 7A, a first shallow trench isolation (STI) region 2 is formed in a memory formation area 100 of the upper part of a semiconductor substrate 1, for example, made of silicon, and a second shallow trench isolation (STI) region 3A is formed in a CMOS formation area 200 of the upper part of the semiconductor substrate 1. In this case, the step height between the first STI region 2 and the semiconductor substrate 1 and the step height between the second STI region 3A and the semiconductor substrate 1 are both, for example, about 100 nm.
Next, as shown in FIG. 7B, an ONO film 6 made of a first silicon oxide film, a silicon nitride film and a second silicon oxide film is entirely formed on the memory formation area 100 and the CMOS formation area 200 of the semiconductor substrate 1.
Next, as shown in FIG. 7C, a bit line (not shown) is formed in the memory formation area 100 of the upper part of the semiconductor substrate 1, and then a part of the ONO film 6 covering the CMOS formation area 200 is removed by wet etching. At this time, the upper part of the second STI region 3A is cut away by wet etching, cleaning and other process steps. As a result, a second STI region 3B is obtained.
Next, as shown in FIG. 8A, a gate dielectric 7 is formed on a part of the semiconductor substrate 1 located in the CMOS formation area 200 by thermal oxidation. In this relation, in the CMOS formation area 200, the upper part of the second STI region 3B is further cut away by cleaning and other process steps also during the formation of the gate dielectric 7. As a result, a second STI region 3C is obtained. Consequently, the step height between the second STI region 3C and the semiconductor substrate 1 is reduced to, for example, about 50 nm.
On the other hand, since in the memory formation area 100 the first STI region 2 is covered with the ONO film 6, the step height between the first STI region 2 and the semiconductor substrate 1 is not reduced. As a result, the step height therebetween is still about 100 nm.
Next, as shown in FIG. 8B, in the memory formation area 100, a memory electrode 8 serving as a gate electrode is formed to cover part of the first STI region 2 across one end thereof, while in the CMOS formation area 200 a CMOS electrode 9 serving as a gate electrode is formed to cover part of the second STI region 3C across one end thereof. In this relation, as described above, the step height between the first STI region 2 and the semiconductor substrate 1 is 100 nm, which is larger than the step height between the third STI region 3C and the semiconductor substrate 1, i.e., about 50 nm.
Next, as shown in FIG. 8C, when the CMOS formation area 200 undergoes a process step of forming a sidewall on a side of the CMOS electrode 9 or other process steps, sidewalls 10 are formed not only on a side of the memory electrode 8 but also on the sides of stepped portions of the first STI region 2 in the memory formation area 10 having a large step height between the first STI region 2 and the semiconductor substrate 1. Therefore, the sidewalls 10 on the stepped portions reduce the surface areas of diffusion layers 11 to be formed in a later diffusion layer formation process step by source/drain (S/D) implantation. At this time, in the CMOS formation area 200, the step height between a part of the second STI region 3C not covered with the CMOS electrode 9 and the semiconductor substrate 1 becomes approximately 0. As a result, a second STI region 3D is obtained.
More particularly, in this process step, in the CMOS formation area 200, the step height between a part of the second STI region 3D located under the CMOS electrode 9 and the semiconductor substrate 1 is approximately 50 nm, and the step height between a part of the second STI region 3D that is not covered with the CMOS electrode 9 and the semiconductor substrate 1 is approximately 0. On the other hand, in the memory formation area 100, the step height between a part of the first STI region 2 located under the memory electrode 8 and the semiconductor substrate 1 is approximately 100 nm, and the step height between a part of the first STI region 2 that is not covered with the memory electrode 8 and the semiconductor substrate 1 is approximately 50 nm.
Values of the step heights between each of the STI regions 2, 3A and the like and the semiconductor substrate 1 are merely given as an example and actually vary within a range of several tens of nm due to variations in process steps. However, the step height between the part of the first STI region 2 located under the memory electrode 8 and the semiconductor substrate 1 and the step height between the part of the second STI region 3D located under the CMOS electrode 9 and the semiconductor substrate 1 have the following relationship: the step height between the first STI region 2 and the semiconductor substrate 1 is always larger than the step height between each of the second STI regions 3C and 3D that are reduced in thickness after process steps in FIGS. 7 and 8 and the semiconductor substrate 1. The reason for this is that the first STI region 2 is covered with the ONO film 6.
By the way, as shown in FIG. 9A, when silicide layers 12 are formed in the semiconductor substrate 1, the sidewalls 10 formed on the stepped portions of the first STI region 2 in the memory formation area 100 reduce the areas of the silicide layers 12. Therefore, when as shown in FIG. 9B contacts 13 are formed on the silicide layers 12 in the memory formation area 100, overlapping margins of the formed contacts 13 and the silicide layers 12 become small.
As described above, according to the known semiconductor device fabricating method, since the step height between the first STI region 2 and the semiconductor substrate 1 in the memory formation area 100 is larger than the step height between the second STI region 3D and the semiconductor substrate 1 in the CMOS formation area 200, the sidewalls 10 formed on the sides of the stepped portions of the first STI region 2 causes the following problems.
First, the surface areas of the diffusion layers 11 formed by source/drain (S/D) implantation are reduced, leading to the increased resistance in the diffusion layers (hereinafter, referred to as “diffusion layer resistance”). In particular, when the silicide layers 12 are formed in the semiconductor substrate 1 and used as thin-wire resistors, the sidewalls 10 formed on the stepped portions of the first STI region 2 of the memory formation area 100 reduce the widths of the silicide layers 12. This increases the thin-wire resistance in the silicide layers 12 (hereinafter, referred to as “silicide thin-wire resistance”).
Second, when the contacts 13 are formed on the diffusion layers 11, the diffusion layers 11 are reduced in their surface areas. This reduces the overlapping margins of the contacts 13 and the diffusion layers 11.